The 21-pin 'MTC' adapter is a more recent standard compared to the 8-pin connector, but is generally much simpler
because you don't normally have any wires from the decoder, so it can be plugged into your loco's 21-pin adapter quickly
and very neatly.
21-pin adapters are common in diesel OO locos, or in some steam locos where the decoder socket is located in the tender.
The pins are arranged in 2 rows of 11 pins each, where one of the rows is missing a pin in one corner. This indicates
which way around the decoder needs to be plugged in - you cannot get it wrong because it wont physically plug in if you
get it wrong!
Be careful to insert the decoder 'square' onto the pins, as they can bend easily if you do it at an angle.
Here is Bachmann's beautiful Wainwright 'C' showing the 21-pin socket in the tender. To fit DCC Sound in this loco
is a piece of cake - 3 screws to take the top of the tender, unplug the DCC blanking plate, solder on the speaker to the
speaker pads provided in the loco, stick the speaker in place (a nice chunky bass-reflex 20x40x10mm 8ohm speaker),
slot on the sound decoder, and that's it!